1. Field of the Invention
The present invention relates generally to the field of pipe and cable detection devices.
2. Related Art
Pipe and cable detection devices, or simply detection devices, perform a number of operations relating to the detection of underground objects. These operations include locating and tracing underground cables, pipes, wires, or other types of conduits. Characteristics of underground objects, such as the depth of the object, the magnitude and direction of an electric current passing through the object, and path of the object, can also be determined by locators. Thus, the routine operations and functioning of underground objects can be monitored and defects in these objects can be easily detected.
Detection devices use radio frequency radiation to detect underground objects and their characteristics. A detection device often includes a transmitter and receiver. In an active mode, the transmitter emits a signal at one or more active radio frequencies. The transmitter can be positioned in different ways to generate a signal that can be used to detect an object. For example, a transmitter can apply a signal to an object through induction, direct connection, or signal clamping. The receiver detects the transmitted signal and processes the detected signal to obtain desired information. In a passive operating mode, the receiver can detect passive radio frequency signals emitted by the underground object: A receiver can also detect a SONDE. A SONDE is self-contained transmitter provided on certain types of underground objects, such as non-metallic objects. Examples of commercially-available pipe and cable detection devices are locators and tools available from Radiodetection, Ltd., a United Kingdom company. Detection devices and tools from Radiodetection, Ltd. include devices such as the PXL-2, PDL-2, HCTx-2, LMS-2, LMS-3, PDL-4, PTX-3, and C.A.T. products.
One particular technique for detecting underground objects, such as the cables, pipes, wires, or other conduits include the use of electronic marker balls. Electronic marker balls are passive tuned LC circuits with different resonant frequencies used to denote the various buried utilities (power, water, sanitary, telephone, gas and cable television). The marker balls, typically enclosed in plastic, are buried along with the utilities at depths of up to two meters below the surface.
In order to detect the electronic marker balls, conventional locators accommodate optional sub-assembly devices for performing locate functions. These electronic marker ball locators function as a ground penetrating radar system, transmitting a burst of electromagnetic energy (principally B-field) at the resonant frequency of the marker ball and processing the reflected signal with respect to any background noise level. Conventional electronic marker ball locators typically process incoming signals using a vertical antenna and pre-amplifier for receiving specific resonant frequency signals associated with the marker ball and include a homodyne mixer to shift the signal spectrum of the received signals down to a band close to direct current level (DC).
The problem with this approach is that signals associated with the mixer drivers couple to the receive antennas, therefore appearing as unwanted in-band signals. This seriously erodes the signal-to-noise (SIN) ratio of the receiver. Although heterodyne mixers have been used as an alternative approach in other applications, a heterodyne mixer would not be appropriate to this application due to the frequency tolerance of the marker balls. Also, in the conventional signal processing approaches, the proximity of the induction loop to the permeable antenna core causes antenna saturation. Another problem is created by the effects of DC offsets in the signal processing path. Finally, some of the conventional approaches also suffer from one-half least significant bit (LSB) noise truncation.
What is needed, therefore, is an electronic marker locator device that can ameliorate the shortcomings of the conventional approaches. Specifically, what is needed is an approach that can provide an improvement to the S/N ratio and prevent unwanted antenna coupling. Additionally, an approach is needed that can reduce the effects of antenna saturation, negate the effects of DC offsets, and minimize the one-half LSB noise truncation.
Consistent with the principles of the present invention as embodied and broadly described herein, an exemplary method for locating electronic marker balls includes receiving a signal representative of a detected marker ball, the signal including resolved in-phase and quadrature-phase (I and Q) components, each signal being centered about a first frequency. Filtering of the I and Q components is provided to (i) shift the first frequency to a second frequency, thus producing second frequency I and Q components and (ii) match the second frequency I and Q components to exponential decay characteristics associated with the marker balls. Next, a phase of the matched second frequency I and Q components is integrated to distinguish the components from noise. The integrating is based upon predetermined gain coefficients and produces integrated I and Q components in accordance with the predetermined gain coefficients. The method also includes determining a magnitude of the integrated I and Q components to produce an I and Q magnitude vector, determining a noise variance associated with the magnitude vector, and adjusting the predetermined gain coefficients in accordance with the determined noise variance.
Features and advantages of the present invention include providing a digital technique to capitalize on signal processing which would be either impossible or impractical in the analogue domain associated with the conventional approaches. Specific advantages include matched filtering for ensuring maximum correlation with the reflected electronic marker ball signal. This improves the S/N ratio by typically 10 dB.
Next, the technique of the present invention facilitates the use of an adaptive Kalman filter to further enhance the S/N ratio by an additional 6 dB. The Kalman filter, as implemented in the present invention, is designed to adapt the 2nd order recursive coefficients in a infinite impulse response (IIR) bi-quadratic filter.
The present invention also facilitates a dual mode locate feature. In dual mode the receiver performs a narrow band locate function simultaneously with an electronic marker ball locate function. This simultaneous operation avoids cross-talk between the two receiving systems, that is, between the electronic marker ball locate system signal coupling to the narrow band and vice versa. In the case of the digital electronic marker ball locate system technique used in the present invention, the marker ball locate system frequency response can be steered away from the continuous wave line frequency. The technique uses an adaptive filter to ensure the narrow band frequency falls on a natural zero in the response of the marker ball locator system receiver. In the present invention, this avoidance is controlled via the decimation ratio used in a low pass filter.